Thermoplastic elastomers are known. They have many of the properties of thermoset elastomers, yet they are processable as thermoplastics. One type of thermoplastic elastomer is a thermoplastic vulcanizate, which may be characterized by finely-divided rubber particles dispersed within a plastic matrix. These rubber particles are crosslinked to promote elasticity.
Certain thermoplastic vulcanizates may advantageously be employed as adhesives. Inasmuch as these compositions are melt processable, these compositions may advantageously be applied as a hot-melt adhesive. While these adhesives have proven to be technologically useful, the olefin-rich nature of many thermoplastic vulcanizate compositions may present adhesion issues with polar surfaces such as metal surfaces.
To improve adhesion to polar surfaces, functionalized polymers have been included in thermoplastic vulcanizate compositions. For example, U.S. Pat. No. 4,957,968 teaches the addition of maleated polypropylene to a thermoplastic vulcanizate composition in order to improve adhesion to metal surfaces.
While the inclusion of these functionalized polymers can improve the adhesive properties of thermoplastic vulcanizates, enhanced adhesion has been discovered with the inclusion of soft plastics. For example, U.S. Pat. No. 6,503,984 teaches thermoplastic vulcanizate compositions that include functionalized polyolefins and non-functionalized polyolefins that are characterized by low flexural modulus and low crystallinity.
As the potential uses of soft thermoplastic vulcanizates that adhere to polar surfaces increases, the demands placed upon these compositions likewise increase. For example, thermoplastic vulcanizate adhesive compositions are often employed to provide a desirable surface to many household tools and appliances. For example, many kitchen utensils include soft, grippable surfaces prepared from thermoplastic vulcanizates. In light of these uses, the adhesion of thermoplastic vulcanizates to polar surfaces, especially after water aging, is of particular importance. Also, while good bond strength may be achieved when adhesive compositions are compression molded to a substrate, which advantageous adhesion may derive from a longer residence wetting time, compression molding techniques are often less efficient than injection molding techniques. The ability to achieve good bond strength when injection molding is not trivial, however, which may derive from the shorter residence wetting time that is typically experienced. Therefore, there is also a desire for hot melt adhesives that show good bond strength after injection molding.